1. Field of the Invention
This invention relates to the field of hydraulic controls for a continually variable transmission.
2. Prior Art
German Patent No. 27 03 487 describes a control valve arrangement for a continually variably transmission. A first regulating valve for determining pressure in the primary servo sets the gear ratio of the belt-drive transmission. A second regulating valve for determining pressure in the secondary servo sets tension in the drive belt. The first of the two regulating valves is acted on as a function of engine speed and torque of the driving engine by a cam disk arrangement actuated by the throttle shaft. The second regulating valve is acted on by a sensor rod connected to the primary servo, whose position is a function of the operating gear ratio of the transmission.
A disadvantage of this control valve arrangement is that the control pressure in the secondary servo is related to the transmitted torque only by a minimum secondary pressure provided for transmitting maximum torque. Consequently, secondary servo pressure is far too high in the underload range where torque is low. Therefore, the components of the transmission, particularly the rotating drive belt, which may be a sliding link belt, is subjected to unnecessarily high load and stress. A fluid pressure pump is required also to produce unnecessarily high pressure, as a result of which the efficiency of the transmission is adversely affected and fuel consumption is increased.
European Patent No. 0158370 describes a control valve arrangement for a continually variable belt-drive transmission in which the control valves are similar to those of the '487 patent. However, patent '370 includes a modulating valve, which varies pressure in the secondary servo as a function of engine torque. The additional modulating valve is acted upon by a siphon diaphragm as a function of vacuum in the intake manifold of the engine. Therefore, in addition to the modulating valve, the siphon diaphragm and corresponding connecting lines are also required. Furthermore, exhaust gas recirculation required in modern engines to lower the level of contaminants in engine exhaust causes intake manifold vacuum to be an unreliable indication of torque delivered by the engine.